Allow verifying different versions of Tails (older versions, testing versions, etc.)

Using our new JSON format and the extension in version 2, we might be able to publish older versions in the JSON file. Or any other ISO/IMG file plus checksum that we want to have in there.

Currently the extension will not check if the version that the user wants to verify is the current one because we publish only the current one, so no mistake can be made here.

But if we decide to publish older versions, we should add a version check and alert the user that they are verifying an old image, and we can even tell them which one, and still allow them to verify the checksum of the old image.

I think this might be useful if we want to allow users to verify RC images for example.

Related issues

• Related to #15995 (closed)

Original created by @u on 16200 (Redmine)